1. Field of the Invention
The present invention relates to a magnetic disk drive and to a head position control method for a magnetic disk drive, and more particularly to a magnetic disk drive and to a head position control method for a magnetic disk drive which are capable of performing high-speed seek and high-accuracy positioning, even if a servo track which is defined by servo information recorded on the surface of the disk is eccentric with respect to the center of rotation.
2. Description of Related Art
In recent years, to achieve an increase in the storage capacity of magnetic disk drives, efforts have been made to achieve even higher density, with an accompanying need to improve access speed. Various methods have been proposed to achieve an increase in magnetic disk drive storage capacity and an improvement in access speed.
In a magnetic disk apparatus (hereinafter referred to simply as a disk drive), a magnetic head (hereinafter referred to simply as a head) records data onto a magnetic disk (hereinafter referred to simply as a disk) by recording a magnetic pattern, and reads out data that has been recorded onto the disk by detecting magnetic field variations occurring due to this recorded magnetic pattern. To establish the position for recording and reading out, a magnetic guide on the disk known as a track is recorded onto the disk coaxially with respect to the center of rotation of the disk. By specifying a track, the position in the radial direction is identified. Each track on the disk is divided in the circumferential direction into a plurality of parts known as sectors, the sector numbers which identify each of the sectors being recorded onto the disk. Recording of data is performed in units of sectors and, by specifying a sector number, it is possible to identify the circumferential position. In a disk drive which has a plurality of disks having the same rotational axis and a head provided for each recording surface of the disk, the heads are linked so as to move in concert, one of the disk surfaces, for example, being made a servo surface, onto which information related to tracks and is recorded, with only data being recorded onto the other disk surfaces, in what is known as a dedicated servo system. However, in this dedicated servo system, the head position with respect to the servo surface has a head position error with respect to the written data position. Because of variations in temperature, vibration of the head movement mechanism, and the like, it is not possible to make this error very small. Therefore, it was difficult to increase the density by making the track spacing sufficiently narrow. For this reason, in recent years data has been written on all disk surfaces, with information related to the track recorded at the very beginning of the sectors of each disk surface, this being detected so as to control the head position. This system, in which servo information is recorded together with data is known as an embedded servo system.
In current disk drives, a disk is mounted on the rotating shaft of a spindle motor, servo information is recorded by means of a servo track writer (STW) so as to perform track writing initialization. The writing and readout of data are performed after performing this initialization operation. In reality, an STW program is started and servo information is written onto the disk surface by a head. Thus, the path of a track is a circle which has as its center the center of rotation. This applies to both the embedded servo system and the dedicated servo system.
However, in reality, because the head and disk are vibrating while the track is being written, the track is not a true circle, but rather vibrates about a circular path. In order to improve the recording density of a disk apparatus, it is necessary to increase the number of tracks that can be recorded on a disk. However, when the disk vibrates, because of the risk of mutual interference between adjacent tracks, it is not possible make the track spacing very narrow, thereby presenting an obstacle to the improvement of recording density.
In consideration of the above-noted drawbacks, the inventor of the present invention envisioned the possibility of improving the recording density by assembling a disk onto a rotational hub after accurately recording the tracks using an external apparatus. If only the track is to be recorded, it is possible to use a highly precise spindle motor and head actuator, it being possible to achieve more accurate track recording by using a dedicated head, enabling the achievement of high recording density by making the track spacing narrow. However, when mounting a disk onto which tracks have been recorded to a rotational hub of a spindle motor, even if the mounting accuracy is improved, it is not possible to avoid some degree of eccentricity, with the center of the track not being in agreement with the center of rotation. Although it is possible to envision that adjustment can possibly make the eccentricity yet smaller, the required adjustment is extremely delicate, it being extremely difficult to achieve a sufficiently accurate adjustment, so that even if this were possible, the labor required to make the adjustment would present the problem of increased cost.
In an embedded servo system having a plurality of disk surfaces, it can be envisioned that it is sufficient to perform tracking control of just the head which is performing data writing or readout, and in the case in which feedback control is used to that the head under control tracks to the center of the track, when the head under control is switched to a different head, the switched head waits until tracking on the target track is achieved, and after tracking is achieved to the target track, data writing and readout are performed. However, this is accompanied by the problem of a lengthened access time when the head is switched.
The present invention was made as a means of solving the above-described problems. An object of the present invention is to implement head position control in a disk drive which is capable of high-speed access, even if a track defined in terms of servo information is eccentric with respect to the center of rotation.
In the various servo information recording regions of a disk surface, servo information including two phase servo bursts which are radially offset and circumferentially staggered are recorded in mutual alternation. The servo information recorded has an indication signal that indicates that it is servo information and a signal which indicates the track number and the sector number. When the disk rotates, the head moves along the periphery thereof, it passing the servo region and detecting and identifying the servo information. As it passes by, if the head overlaps the two phase servo bursts in the same way, the strength of the signals detected by the head from the two phase servo bursts will be the same, so that it is possible to know that the head is passing between the two phase servo bursts. In the case in which the head is displaced toward one of the two phase servo bursts, so that there is a ratio of strengths of the signal detected from the two phase servo bursts changes, it is possible to know toward which servo information the head is displaced. In servo systems in the past, feedback control was performed so that the head passed midway of the two phase servo bursts. Because control was performed so that the head moved along the circular servo track, the head moved along this circular servo track in a data region as well, and data recording was also performed along this circular servo track.
As noted above, while in the servo system of the past control was performed so that the head passed midway of the two phase servo bursts, where the strengths of the signals detected by the head from the two phase servo bursts were the same, it is also possible to perform control for each sector individually, so as to control the ratio of strengths of the signal detected by the head from the two phase servo bursts, so that the head describes a prescribed path. The inventor of the present invention, focusing on this, made it possible to perform control so that the head moves along a circular rotational path, which is a hypothetical circular path concentric with the rotation of the disk, even if the circular path (servo circular path) defined by the servo information recorded on the disk is eccentric with respect to the center of rotation. Even in the case of a combination of a plurality of disks and heads, because each head is controlled so as to move along a circular rotational path, access control is possible in the same manner as when there is no eccentricity. By doing this, even in the case in which a disk which has had precise tracks recorded onto it is assembled, it is possible to achieve a disk drive capable of high-speed access and high-accuracy positioning without the need to adjust the eccentricity to zero.
Further, when the head is controlled to move along a hypothetical circular path, the head is ideally retained to be stable. Therefore, a head positioning mechanism does not consume power. Then, power consumption of the disk drive can be reduced.